Decentralized sharing of information and data through direct exchange may be used to distribute storage capacity and load across a network of peers. In fully decentralized peer-to-peer networks there is no need for a central coordinator; rather, communication may be handled individually by each peer.
In a peer-to-peer network, a node becomes a member of the network by establishing (possibly on a purely ad-hoc basis, or possibly in response to a message from a remote coordinator) a connection with at least one peer currently in the network. Each node typically maintains a small number of connections with its peers and messages are sent from one peer to another with each peer responding to queries for information that it may have stored locally. For example, to search for a file, a node broadcasts a search request to its directly connected peers, those directly connected peers propagate the requests to their own peers, and so on. Each such hop from one peer to the next contributes to an increase in the bandwidth on the communication links and furthermore to the time required to get results for the queries. The network bandwidth utilized for a search request is proportional to the number of messages, which in turn is dependent upon the number of peers that must process the request before the requested data has been located.
In a typical peer-to-peer file-searching network, each peer node has known bandwidth constraints and an arbitrary list of potential peer connections (either all registered members of the network, or an arbitrary subset), and peers are connected and disconnected from each other based on arbitrary criteria. The node has an arbitrary list of peers in the network and sends Ping messages to those peers to determine which are alive. A connection is established only if the other peer is active and responds with a Pong message. Therefore, a Gnutella node typically establishes connections with the first peers it finds available. To search for a particular file or document, the node sends a search request to its connected peers and those peers in turn forward the request to their own peers and so on. Each peer in the sending path keeps track of each request it forwards and returns any received results to the upstream requestor. Therefore, the number of peers that process the request grows exponentially as the request is propagated to ever more distant peers, with the maximum number of permitted hops (the message “Horizon”) being set by the originating node and is typically about seven times. The number of connections maintained by a given peer in the known peer-to-peer Gnutella networks is based on the network bandwidth available to the peer and the actual bandwidth utilized for propagating queries to other peers; a peer may arbitrarily drop one or more of its existing connections if the bandwidth utilization by query messages gets too high.